With the Federal Reserve committing to buy $85 billion worth of securities every more -- including $40 billion worth of mortgages -- until the market recovers, while also announcing it will keep interest rates at extreme but artificially low rates until at least 2015, the market jumped 206 points, or 1.6%. But some companies couldn't come to the party and instead fell, some even dropping by double-digit percentages.

So let's see whether they had good reason to tumble, as sometimes panic-fueled declines can actually lead to excellent buying opportunities.

For crying out loud
Online review site Yelp (NYSE: YELP) fell more than 7% toward the end of last week even as analyst initiated covering with a buy recommendation. Believing the site is "proving disruptive [and] scalable" and that it "should over time prove highly profitable," the Jefferies analyst was perplexed by the decline. Certainly the announcement Yelp made that it was launching in Singapore would suggest that the analyst's arguments make sense, but there may be a deeper issue that isn't resonating with investors: the fake review.

The case of Scott Van Duzer underscores how companies that rely on social networking are at the mercy of forces beyond their control, which isn't good. The Florida pizzeria owner who gave President Obama a bear hug of support saw his Yelp page inundated with bad reviews, and while there has been a groundswell of support for him in the aftermath, it highlights the questionability of what it is you're reading.

It's been noted that even Amazon.com is subject to fake reviews, but following as it does the news that maybe more than 80 million Facebook (NYSE: FB) accounts are fake and that Twitter is probably populated with more popularity-bots than real followers, it calls into question just how sustainable the social-media business model is.

I actually think it's less of a problem for a Yelp than for Facebook, as it will probably only have such high-profile instances on rare occasions, but I also think the stock is more just part of a social-networking bubble that won't last, which is why I've rated Yelp to underperform the markets on Motley Fool CAPS. But let me know in the comments section below if you think there's a reason Yelp should be able to grow beyond its current valuation.

Pay up
Medical billing services provider Accretive Health (NYSE: AH) had more tangible problems, as Minnesota regulators charged that it engaged in aggressive collection practices that violated federal patient protection laws. No one likes bill collectors calling during dinner, but Accretive was alleged to have begun dunning patients in the ER who hadn't even been triaged yet. While the hospital, which is no longer affiliated with Accretive, was probably not going to be penalized for the violations, as it will be given a chance to clean up its act, it also raised the specter that this is a pervasive pattern and part of Accretive's business practices.

Earlier this summer, the bill collector agreed to settle a state case against it that led to a $2.5 million fine and an agreement not to do business in Minnesota for two years. With Illinois also trying to get it to stop its bill collecting in that state, it may find fewer hospitals elsewhere that want to be associated with it.

Even so, Wall Street apparently believes that despite all else, it remains effective at what it does, and of the analysts that CAPS tracks, four have weighed in on the stock and believe it will outperform the indexes. You can offer up your own opinion on Accretive Health and its collection practices in the comments box below.

Running out of power
Previously I likened Skullcandy (Nasdaq: SKUL) to being the "Kim Kardashian of headphones," as it offered investors little more than a vapid chance to cash in on its marketing prowess since it had no competitive moat to protect its business. Now it appears even analysts who were enamored of its dangerous curves think the headgear maker is little more than eye candy. Morgan Stanley says channel checks show retailers offering big discounts on its headgear to move product, as Dr. Dre's Beats brand is stealing far more share than anticipated, and the stock tumbled 16% on the report.

Moreover, Apple (Nasdaq: AAPL) announced an improved earbud when it unveiled the iPhone 5, which places another competitive stumbling block in front of it. I still maintain its best hope is to be bought out by Sony, JVC, or some other sound specialist, but as CAPS member davinci76 points out, with Skullcandy's extremely high short interest ratio -- days to cover surged to more 23 in the most recent reporting period -- any bit of good news could set the stock bouncing higher if short sellers fear they're being squeezed.

That doesn't make for a good long-term investment, though, so I'm maintaining my underperform rating on CAPS, but Skullcandy still has its adherents, so let me know below if you think its future will be more  lasting than cotton candy on the tongue.

Ready for a resurrection
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